A measuring cell of this general type has become known from U.S. Pat. No. 4,358,679. In the prior-art measuring cell, the gas sample to be analyzed enters a measuring path, which has two limiting surfaces located opposite each other. The first limiting surface is designed as a spherical mirror, and the second limiting surface is transparent to infrared radiation. An infrared radiation source and a radiation receiver are arranged at the second limiting surface such that a measuring beam emitted by the infrared radiation source is reflected on the mirror and then reaches the radiation receiver. The measuring beam now passes through the measuring path filled with the gas sample to be analyzed twice. The prior-art measuring cell has a length of about 7.5 cm and a width of about 3 cm and is suitable for the detection of carbon dioxide and carbon monoxide. The design of the measuring cell is especially advantageous for portable devices, because the electrical components, such as the radiation source and the radiation receiver, are fastened on one side of the measuring path and thus they can be connected to an electrical evaluating circuit in an especially simple and compact manner. In contrast, only optical components, namely, the spherical mirror, are arranged on the opposite side of the measuring path.
Difficulties arise in connection with the detection of gas components, e.g., methane, which require a markedly longer absorption path. The size of the measuring cell is, in general, not critical in the case of permanently installed devices, so that the necessary absorption path is reached with a correspondingly more largely dimensioned measuring path with a single reflection of the measuring beam.